1. Field of the Invention
The present invention relates to a power supply device for supplying a DC voltage to various electronic devices and specifically to a power supply device which includes a step-up converter.
2. Description of the Prior Art
Step-up converters have highly-efficient power conversion characteristics and have been widely used in recent years as power supply devices in various electronic devices which operate on a battery used as a DC input power supply. A common step-up converter includes an inductor with an end connected to a DC input power supply, a switch connected between the other end of the inductor and a reference voltage node, a diode with an anode connected to the other end of the inductor, and a capacitor connected between the cathode of the diode and the reference voltage node. The inductor is charged/discharged by repeating turning on/off the switch, so that the voltage is boosted. The boosted voltage is accumulated in the capacitor and then output as a DC output voltage.
In the step-up converter having such a common structure, when the output side is short-circuited or overloaded, an overcurrent undesirably flows from the DC input power supply to the output side via the inductor and diode even if the switch is stopped such that the step-up operation is suppressed. Conventionally, to avoid damage to parts by such an overcurrent, a current detection resistor and a transistor for constant current control are inserted between the diode and output terminal of the step-up converter. When the current detection resistor detects an overcurrent, the transistor is controlled to have a constant current. With this feature, the output current is maintained constant even when the output side is short-circuited or overloaded. Thus, the inductor and diode are protected.
However, in the case of the aforementioned step-up converter having the overcurrent protection function, the current detection resistor and the transistor for constant current control need to be sufficiently resistant to a supply current which is supplied to a load in a normal operation. Insertion of the current detection resistor and the transistor for constant current control in a current supply route leads to occurrence of a conduction loss, which deteriorates the conversion efficiency of the step-up converter.